GPS, or Global Positioning System, is funded by and controlled by the U. S. Department of Defense (DOD). While there are many thousands of civil users of GPS worldwide, the system was designed for and is operated by the U. S. military. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity, and time. Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock. The GPS satellite is one of a constellation of approximately twenty-four GPS satellites operated by United States Department of Defense orbiting the Earth approximately twice every twenty-four hours at an altitude of approximately ten thousand miles and traversing the sky above a location on the surface of the Earth in approximately four hours.
Another satellite-based system called Global Orbiting NAvigational SyStem (GLONASS) is being developed and deployed by Russia. The basic goals, functions, and capabilities of the GLONASS are similar to the GPS. Although the present discussion focuses on a GPS-based system, a GLONASS-based system can be used for a similar function with a similar result.
GPS receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of GPS (or NAVSTAR) satellites. These satellites transmit, as part of their message, both satellite positioning data as well as data on clock timing, so-called “ephemeris” data.
The GPS satellites transmit two microwave carrier signals. The L1 frequency (1575.42 MHz) carries the navigation message and the Satellite Positioning Service (SPS) code signals. The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by Precise Positioning System (PPS) equipped receivers. There are two principal functions of GPS receiving systems: (1) computation of the pseudoranges to the various GPS satellites, and (2) computation of the position of the receiving platform using these pseudoranges and satellite timing and ephemeris data. The pseudoranges are simply the time delays measured between the received signal from each satellite and a local clock. The satellite ephemeris and timing data is extracted from the GPS signal once it is acquired and tracked.
Virtually all known GPS receivers utilize correlation methods to compute pseudoranges. These correlation methods are performed in real time, often with hardware correlators. GPS signals contain high rate repetitive signals called pseudorandom (PN) sequences. The codes available for civilian applications are called C/A codes, and have a binary phase-reversal rate, or “chipping” rate, of 1.023 MHz and a repetition period of 1023 chips for a code period of 1 msec. The code sequences belong to a family known as Gold codes. Each GPS satellite broadcasts a signal with a unique Gold code.
For a signal received from a given GPS satellite, following a downconversion process to baseband, a correlation receiver multiplies the received signal by a stored replica of the appropriate Gold code contained within its local memory, and then integrates, or lowpass filters, the product in order to obtain an indication of the presence of the signal. This process is termed a “correlation” operation. By sequentially adjusting the relative timing of this stored replica relative to the received signal, and observing the correlation output, the receiver can determine the time delay between the received signal and a local clock. The initial determination of the presence of such an output is termed “acquisition.” Once acquisition occurs, the process enters the “tracking” phase in which the timing of the local reference is adjusted in small amounts in order to maintain a high correlation output. The correlation output during the tracking phase may be viewed as the GPS signal with the pseudorandom code removed, or, in common terminology, “despread.” This signal is narrow band, with bandwidth commensurate with a 50 bit per second binary phase shift keyed data signal which is superimposed on the GPS waveform. The correlation acquisition process is very time consuming, especially if received signals are weak. To improve acquisition time, newer military GPS receivers are likely to utilize a multiplicity of correlators that allows a parallel search for correlation peaks.
The ability to track low power GPS signals is important for a number of real-time applications, including cases where the GPS signal may be attenuated, jammed, or subject to interference. Previous approaches to these obstacles have included to varying degrees (i) signal processing to enhance sensitivity, (ii) controlled radiation pattern antennas (CRPAs) to thwart jamming, and (iii) control of the receiving environment, if possible, to ward off interference. Unfortunately, in many of these cases, only limited performance improvement is feasible due to practical constraints.
In the case of (i) signal processing, the fundamental limit to increased performance is established by the data bit boundaries in the GPS message. The intrinsic GPS data broadcast rate is 50 bits per second. Ordinary receivers cannot integrate the signal across these 20 ms intervals. Extension of the integration interval would actually lead to a decrease in performance because the data bits will appear as random noise that averages to zero. Therefore, a general practical limit is 20 ms averaging.
Some practitioners have tried to push this limit by squaring the GPS signal. However, squaring is an inefficient means of recovering information because the noise is mixed with itself, resulting with a significant baseband noise component superimposed over the squared signal at baseband. Other techniques have been devised that employ data stripping—the local application of limited a priori knowledge of the GPS bit sequence that takes advantage of a tendency in GPS to often repeat the same sequence multiple times—in order to remove the GPS data and obtain longer integration times. Unfortunately, this technique can often provide marginal results—especially in critical applications. The main shortcoming is that it completely falls apart when the GPS message changes, and this occurrence is frequent and unpredictable.
Prior processing efforts have also taught away from the current invention of employing feed-forward data to enhance performance. For example, U.S. Pat. No. 6,133,874 teaches that, “Coherent integration beyond 20 milliseconds is normally inadvisable since the presence of a priori unknown 50 baud binary phase shift keyed data (the satellite data message) placed on top of the signal does not allow coherent processing gain beyond one data bit period, or 20 milliseconds.” Similarly, U.S. Pat. No. 5,664,734 explains, “If the carrier frequency and all data rates were known to great precision, and no data were present, then the signal-to-noise ratio could be greatly improved, and the data greatly reduced, by adding to one another successive frames . . . . The presence of 50 baud data superimposed on the GPS signal still limits the coherent summation of PN frames beyond a period of 20 msec.”
Null steering antennas (ii) reduce jamming by identifying the direction of origin of a jammer, then spatially notching out all signals in that direction. What remains is the unjammed GPS signals that come from other directions. With additional phased array electronics, it is also possible to create more tightly focused beams on individual GPS satellites, thereby increasing signal strength. While the jamming protection of CRPAs is excellent, such antennas are often heavy, bulky, and expensive.
Last, controlling the receiving environment to minimize interference sources (iii) is often involved because it must be carried out under a regulatory regime. For example, ultrawideband (UWB) devices have already been shown to interfere with certain GPS devices on occasion. There is a general desire to have both of these devices coexist—the potential user base is fundamentally the same. In the best of all worlds, a regulatory environment will exist that will enable UWB to coexist with GPS and other incumbent bands. However, in spite of best efforts to create and conform to such a regulatory environment, there will always be deviations that create exceptions to proper performance. It is for these cases that the invention described herein is likely to be most useful, namely, as a “safety net” against unexpected interference.
What is needed is a system that provides robust GPS performance under any of these adverse conditions with a simple solution that does not incur the associated penalties in terms of size, weight, power, and cost.